Chapter 118: USMI

Question 1

What are the (4) interacting mechanisms which are responsible for maintaining urinary continence?

Answer 1

• Urethral smooth muscle (most prevalent in the cranial 1/2 of the urethra)
• Striated muscle (urethral “sling” in caudal third of the urethra)
• Natural elasticity of the urethral wall
• Physical properties of the urethra (length and diameter, pelvic diaphragm muscle mass)

Question 2

What is the role of the pudendal nerve in continence?

Answer 2

Contracts striated muscle of the external urethral sphincter for voluntary control of micturition.

Question 3

List potential causes of congenital USMI?

Answer 3

• Abnormally short or absent urethra (esp. cats)
• Diverticula and dilations in juvenile male dogs

Question 4

What factors are known or suspected to contribute to USMI?

Answer 4

• Urethral tone
• Tail docking (through reduction in pelvic diaphragm mass)
• Urethral length
• Bladder neck position (changes in conveyance of abdominal pressure to the urethra)
• Body size and breed
• Gonadectomy
• Hormones

Question 5

What breeds are overrepresented with USMI?
How much more likely is USMI to occur in large/giant breeds?

Answer 5

• Old English Sheepdog
• Doberman
• Rottweiler
• Weimeraner
• Irish Setters

Large- and giant-breed dogs are 7x more likely to develop USMI

Question 6

How does gonadectomy predispose to the development of USMI?

Answer 6

• Results in significantly higher proportion of collagen and lower proportion of muscle in the lower urinary tract. (Female urethra is already about 75% collagen)
  -Possibly through urothelium/urethral lining atrophy from lack of estrogen

Risk of developing incontinence is 8x higher in spayed females

Question 7

How do hormones effect USMI?

Answer 7

• Lack of estrogen results in reduction of smooth muscle tone
• Gonadectomy may also reduce the numbers of muscle fibers of the striated urethralis muscle
• Increase in gonadotropins LH and FSH leads to lower expression of COX-2 and its m-RNA, leading to decreased prostaglandins in the lower urinary tract

Question 8

How is USMI diagnosed?

Answer 8

Mostly a diagnosis of exclusion. Need to rule out other causes of incontinence such as ectopic ureters or conformational abnormalities causing overflow (recessed vulva)

Question 9

List the (3) main options for medical management of USMI:
What is the rate of single-therapy resulting in cure of incontinence?

Answer 9

Phenylpropanolamine (Proin)- alpha adrenergic agonist
Estrogens (DES) - Improve smooth muscle contractility and sensitivity to alpha-adrenergic stimulation
GnRH analogs - decrease pituitary release of LH and FSH. Action suspected to be mainly on bladder function

50% of dogs will be cured with single therapy treatment

Question 10

When are estrogens contraindicated in the treatment of USMI?

Answer 10

Prior to the first estrus
- About 50% of dogs will have their incontinence resolve after the first estrus
- Contraindicated before due to potential adverse feedback on the pituitary

Question 11

How does treatment of USMI in male dogs differ?

Answer 11

• Less responsive to medical management
• Most responsive to alpha-adrenergics
• Vas deferensopexy and prostatopexy have been reported
• Artificial sphincter placement in three dogs and appears to be effective